This signer doesn't create input scripts for tx inputs. Instead it expects inputs to contain scripts with
empty sigs and replaces one of the empty sigs with calculated signature.

This signer is always implicitly added into every wallet and it is the first signer to be executed during tx
completion. As the first signer to create a signature, it stores derivation path of the signing key in a given
ProposedTransaction object that will be also passed then to the next signer in chain. This allows other
signers to use correct signing key for P2SH inputs, because all the keys involved in a single P2SH address have
the same derivation path.